1. Field of the Invention
The present invention relates to an acoustic wave device and a method of manufacturing the acoustic wave device, in particular, relates to an acoustic wave device that has an electrode connected to an acoustic wave element and a method of the acoustic wave device.
2. Description of the Related Art
An acoustic wave device is widely used as a signal filter of an electrical and electronic device using an electromagnetic wave. The acoustic wave device is used as a transmitting and receiving filter of a radio communication device such as a mobile phone or a frequency filter for visual such as a television or a videotape recorder. A surface acoustic wave element or a boundary acoustic wave element is used as an acoustic wave element. A surface acoustic wave element has a structure in which an electrode exciting a surface acoustic wave such as a comb electrode is provided on a piezoelectric substrate made of LiNbO3, LiTaO3 or the like. It is necessary to form a space on the piezoelectric substrate and on the electrode, because the surface acoustic wave propagates on a surface of the piezoelectric substrate. It is necessary to seal the acoustic wave element in order to gain trust, because there is not provided a thick protective membrane on the piezoelectric substrate and on the electrode.
On the other hand, a boundary acoustic wave element has a dielectric material on a comb electrode, and uses a boundary acoustic wave propagating between a piezoelectric substrate and the dielectric material. A space on the comb electrode may not be formed. The acoustic wave device may have an external connection terminal for inputting and outputting an electrical signal from and to outside. In the structure, an electrode is formed with a plating method, if a thick electrode is necessary as an external connection terminal.
As shown in FIG. 1A through FIG. 2B, a description will be given of a conventional embodiment in which a plated electrode connected to a surface acoustic wave element or a boundary acoustic wave element is formed. As shown in FIG. 1A, a comb electrode 12 is formed on a piezoelectric substrate 10. A conductive pattern 14 connected to the comb electrode 12 is formed on an electrode region 44 where a plated electrode is to be formed. A first conductive pattern 18 is formed on a cutting region 42 for individuating a wafer. A barrier layer 16 is formed on the conductive pattern 14 in the electrode region 44. A protective membrane 20 is formed on the comb electrode 12. A cavity 60 is formed on a functional region 40 of a surface acoustic wave element. A sealing portion 23 (a first sealing portion 22 and a second sealing portion 24) is formed on the piezoelectric substrate 10 so that the electrode region 44 acts as a non-covered portion 54.
As shown in FIG. 1B, a seed layer 38 is formed on the second sealing portion 24 and in the non-covered portion 54. As shown in FIG. 1C, a photo resist 39 is formed on the seed layer 38 except in the non-covered portion 54. As shown in FIG. 1D, an inner side of the non-covered portion 54 is plated when an electrical current is provided to the inner side of the non-covered portion 54 via the seed layer 38. This results in a formation of a plated electrode 28.
In the conventional embodiment, it is preferable that the plated electrode 28 is formed with a plating method, because the plated electrode 28 passes through the sealing portion 23 having a cavity. It is necessary to provide an electrical current to a pattern where a plated electrode is to be formed if an electroplating method having a high plating speed is used. And so, the seed layer 38 is provided on the sealing portion 23 as shown in FIG. 1B.
Japanese Patent Application Publication No. 11-26394 and Japanese Patent Application Publication No. 2004-56036 disclose a method of forming a plated electrode with a seed layer as shown in the conventional embodiment.
The conventional embodiment needs complicated steps of forming the seed layer 38 as shown in FIG. 1B, forming the photo resist for selectively plating as shown in FIG. 1C, removing the photo resist 39 as shown in FIG. 2A and removing the seed layer 38 as shown in FIG. 2B. There is a case where the piezoelectric substrate 10 is damaged when the seed layer 38 or the photo resist 39 is removed. Further, there is a case where the acoustic wave element is broken or degraded, because a voltage is implied to metal patterns such as the comb electrode 12 on the piezoelectric substrate 10 when the piezoelectric substrate 10 is subjected to a stress in the manufacturing process. For example, in a case where the sealing portion 23 having a large thickness is on the piezoelectric substrate 10 as shown in the conventional embodiment, the stress subjected to the piezoelectric substrate 10 is enlarged and the acoustic wave element tends to be broken.